Application Note 5296
Authors: G.E. Theriault, A.J. Leidich, and T.H. Campbell
Application of the CA3018 Integrated-Circuit Transistor
Array
The CA3018 integrated circuit consists of four silicon epitaxial
transistors produced by a monolithic process on a single chip
mounted in a 12-lead TO-5 package. The four active devices,
two isolated transistors plus two transistors with an
emitter-base common connection, are especially suitable for
applications in which closely matched device characteristics
are required, or in which a number of active devices must be
interconnected with non-integrable components such as tuned
circuits, large-value resistors, variable resistors, and
microfarad bypass capacitors. Such areas of application
include if, rf (through 100MHz), video, agc, audio, and DC
amplifiers. Because the CA3018 has the feature of device
balance, it is useful in special applications of the differential
amplifier, and can be used to advantage in circuits which
require temperature compensation of base-to-emitter voltage.
amplifier. In pulse video amplifiers and line-driver circuits, Q4
can be used as a forward-biased diode in series with the
emitter of Q3. Q3 may be used as a diode connected to the
base of Q4; in a reverse-biased connection, Q3 can serve as a
protective diode in RF circuits connected to operational
antennas. The presence of Q3 does not inhibit the use of Q4 in
a large number of circuits.
In transistors Q1, Q2, and Q4, the emitter lead is interposed
between the base and collector leads to minimize package
and lead capacitances. In Q3, the substrate lead serves as the
shield between base and collector. This lead arrangement
reduces feedback capacitance in common-emitter amplifiers,
and thus extends video bandwidth and increases tuned-circuit
amplifier gain stability.
Circuit Applications
The applications for the CA3018 are many and varied. The
typical applications discussed in this Application Note have
been selected to demonstrate the advantages of four matched
devices available on a single chip. These few examples should
stimulate the generation of a great many more applications.
Video Amplifiers
A common approach to video-amplifier design is to use two
transistors in a configuration designed to reduce the feedback
capacitance (appearing as a Miller capacitance) inherent in a
single triode device. Three configurations which utilize two
devices are (1) the cascode circuit, (2) the single-ended
differential-amplifier, and (3) the common-collector,
common-emitter circuit. In all three circuits, the output-to-input
feedback capacitance is minimized by isolation inherent in the
configuration. The availability of four identical transistors in a
common package provides a convenient vehicle for these
circuit configurations for video-amplifier design. Two of the
many possible circuit variations are discussed in the following.
BROADBAND VIDEO AMPLIFIER
FIGURE 1. SCHEMATIC DIAGRAM AND T0-5 TERMINAL
CONNECTIONS FOR THE CA3018 INTEGRATED
CIRCUIT TRANSISTOR ARRAY
Circuit Description and
Operating Characteristics
The circuit configuration for the CA3018 is shown in Figure 1.
In a 12-lead TO-5 package, because it is necessary to provide a
terminal for connection to the substrate, two transistor
terminals must be connected to a common lead. The
particular configuration chosen is useful in emitter-follower
and Darlington circuit connections. In addition, the four
transistors can be used almost independently if terminal 2 is
grounded or AC grounded so that Q3 can be used as a
common-emitter amplifier and Q4 as a common-base
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A broadband video amplifier design using the CA3018 is
shown in Figure 2. This amplifier may be considered as two
DC-coupled stages, each consisting of a common-emitter,
common-collector configuration. The common-collector
transistor provides a low-impedance source to the input of the
common-emitter transistor and a high-impedance,
low-capacitance load at the common-emitter output. Iterative
operation of the video amplifier can be achieved by capacitive
coupling of stages.
Two feedback loops provide DC stability of the broadband
video amplifier and exchange gain for bandwidth. The
feedback loop from the emitter of Q3 to the base of Q1
provides DC and low-frequency feedback; the loop from the
collector of Q4 to the collector of Q1 provides both DC feedback
and AC feedback at all frequencies.
CAUTION: These devices are sensitive to electrostatic discharge; follow proper IC Handling Procedures.
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Application Note 5296
The frequency response of the broadband video amplifier is
shown in Figure 3. The upper 3dB break occurs at a frequency of
32MHz. The low-frequency 3dB characteristics are determined
primarily by the values of capacitors C1, C2, and C3. The
low-frequency 3dB break occurs at 800Hz. The mid-frequency
gain of 49dB is constant to within 1dB over the temperature
range from -55° to -125°C. The upper 3dB break is constant at
32MHz from -55°C to +25°C, and drops to 21MHz at +125°C.
provide a low output impedance to maintain bandwidth for
iterative operation.
FIGURE 4. SCHEMATIC DIAGRAM FOR A CA3018 CASCODE VIDEO
AMPLIFIER
FIGURE 2. SCHEMATIC DIAGRAM FOR A CA3018 BROADBAND
VIDEO AMPLIFIER
The frequency response of the cascode video amplifier is shown
in Figure 5. The lower and upper 3dB points occur at frequencies
of 6kHz and 11MHz, respectively. The lower 3dB point is primarily
a function of capacitors C1, C2, and C3. The upper 3dB point is a
function of the devices and of the load resistor R1, and is
10.5MHz at -55°C and 5MHz at 125°C.
The total power dissipation over the entire temperature range is
22.8mW. The DC output voltage varies from 2.33V at -55°C to 3V
at +125°C. The tangential sensitivity occurs at 20µVP-P. The
dynamic range is from 20µVP-P to 4mVRMS at the input.
The circuit of Figure 2 demonstrates a typical approach that can
be altered, especially with regard to gain and bandwidth, to meet
specific performance requirements.
FIGURE 5. VOLTAGE GAIN AS A FUNCTION OF FREQUENCY FOR THE
CASCODE AMPLIFIER OF FIGURE 4
FIGURE 3. VOLTAGE GAIN AS A FUNCTION OF FREQUENCY FOR THE
BROADBAND VIDEO AMPLIFIER OF FIGURE 2
CASCODE VIDEO AMPLIFIER
The cascode configuration offers the advantages of
common-emitter gain with reduced feedback capacitance and
thus greater bandwidth. Figure 4 shows a typical circuit diagram
of a cascode video amplifier using the CA3O18. Transistors Q2
and Q1 comprise the common-emitter and common-base
portions of the cascode, respectively. The common-base unit is
followed by cascaded emitter followers (Q3 and Q4) which
2
The mid-frequency voltage gain of the amplifier is 37dB ±1dB
over the temperature range from -55°C to 125°C. The power
dissipation varies from 16.8mW at -55°C to 17.6mW at 125°C.
The amplifier has a tangential sensitivity of 40µVP-P and a useful
dynamic input range from 40µVP-P to 16.6mVP-P.
15MHz RF Amplifier
Figure 6 shows a typical design approach for a tuned amplifier
for use in the frequency range of 2 to 30MHz in military receivers.
This circuit was designed for a mid-band frequency of 15MHz to
demonstrate its capability. Gain is obtained in a common-emitter
stage (Q4). Transistor Q2 is used as a variable resistor in the
emitter of Q4 to provide improved signal-handling capability with
age. Transistor Q1 is used as a bias diode to stabilize Q4 with
temperature, and the reverse breakdown of Q3 as a diode is used
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to protect the common-emitter stage from signal overdrive of
adjacent transmitters.
FIGURE 7. IN-BAND CROSS-MODULATION CHARACTERISTIC OF
15MHz AMPLIFIER OF FIGURE 6 (DATA TAKEN WITH
UNTUNED INPUT)
Final If Amplifier Stage and Second Detector
FIGURE 6. SCHEMATIC DIAGRAM FOR A CA3018 15MHz RF
AMPLIFIER
The tuned-circuit design of Figure 6 utilizes mismatching to
obtain stability. Although the usable stable gain for a
common-emitter amplifier using this type of transistor is 26dB at
15MHz, the tuned RF amplifier was designed for a total gain of
20dB to obtain greater stability and more uniform performance
with device variations. The general performance characteristics
of the circuit are as follows:
Power Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20dB
Power-Gain Variation from -55 to ±125°C . . . . . . . . . . . . . . ±1dB
Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 315kHz
Noise Figure at Full Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4dB
AGC Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45dB
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8mW
Figure 8 illustrates the use of the CA3018 as a last If amplifier
and second detector (0.1V emitter voltage on terminal 1). The
bias on transistor Q4 is maintained at approximately cutoff to
permit the cascaded emitter-follower configuration (Q3 and Q4)
to be used as a second detector. Because this stage is driven by a
common collector configuration, the input impedance to the
detector can be kept high. A low output load impedance can be
used as a result of the output current capability of the cascaded
emitter-follower configuration. The input impedance (terminal 9)
of approximately 9000Ω is largely determined by the bias
network. A minimum If input power of 0.4µW must be delivered
to terminal 9 for linear operation. The audio output power for 60
percent modulation for this drive condition is 0.8µW. Linear
detection is obtained through an input range of 20dB for 60
percent modulation. This detector arrangement requires less
power output capability from the last If amplifier than a
conventional diode detector yet allows a low DC load resistor to
achieve a good AC-to-DC ratio for the first audio amplifier.
Figure 7 shows the cross-modulation characteristics of the circuit
for in-band signals. For out-of-band undesired signals, the
cross-modulation performance is improved by the amount of
attenuation provided by the input tuned circuit. Cross-modulation
performance also improves (i.e. more interfering signal voltage is
required for cross-modulation distortion of 10 percent) with
increased agc as a result of the degeneration in the emitter of
Q4.
FIGURE 8. SCHEMATIC DIAGRAM FOR A CA3018 FINAL IF
AMPLIFIER AND SECOND DETECTOR
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The If amplifier of Figure 8 has a voltage gain of 30dB at 1MHz.
Transistor Q1 is used in the base-bias loop of the
common-emitter amplifier Q2 to stabilize the output operating
point against temperature variations. This arrangement also
eliminates the need for an emitter resistor and bypass capacitor,
and thus provides a larger voltage-swing capability for Q2. If Q2 is
biased conventionally with base-bias resistors, Q1 can be made
available for the first audio or agc amplifier.
Class B Amplifier
Characteristics were obtained on a low-level class B amplifier to
establish the idling-current performance of nearly identical
devices on a single chip with respect to temperature variations.
The transistors in the CA3018 can be used only for low-power
class B operation (maximum output of 40mW) because of the
hFE roll-off and moderately high saturation resistance at high
currents. A typical circuit is shown in Figure 9. Idling-current bias
is provided to Q1 and Q2 by use of transistor Q3 as a diode (with
collector and base shorted) and connection of a series resistor to
the supply. The idling current for each transistor in the class B
output is equal to the current established in the resistance-diode
loop. Because the resistor R1 is the predominant factor in
controlling the current in the bias loop, the bias current is
relatively independent of temperature. In addition, because the
devices have nearly equal characteristics and are at the same
temperature, the idling current is nearly independent through the
full military temperature range. The total idling current for
transistors Q1 and Q2 in Figure 9 varies from 0.5 to 0.6mA from
-55 to +125°C. Excellent balance between output devices is
achieved throughout the range.
FIGURE 9. SCHEMATIC DIAGRAM FOR A CA3018 CLASS H
AMPLIFIER
AC feedback as well as DC feedback can be obtained by
substitution of two resistors R2 and R3 in place of R1, as shown
by the dotted lines in Figure 9. These two resistors, which have a
parallel combination equal to R1. are connected between
collector and base of transistors Q2 and Q1. The added feedback
reduces the power gain by approximately 6dB (30 to 24dB), but
improves the linearity of the circuit. Although the output-power
capability for the circuit shown in Figure 9 is approximately
18mW, output levels up to 40mW can be obtained in similar
configurations with optimized components.
FIGURE 10. SCHEMATIC DIAGRAM FOR A CA3018 100MHz
CASCODE AMPLIFIER
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100MHz Tuned RF Amplifier
Figure 10 illustrates the use of the CA3018 in a 100MHz cascode
circuit with an agc amplifier. Transistors Q1 and Q2 are used in a
cascode configuration, and transistors Q3 and Q4 are used to
provide an agc capability and amplification. With a positive-going
agc signal, current in the cascode amplifier is transferred to the
Darlington configuration by differential-amplifier action. This agc
amplifier has the advantage of low-power drive (high input
impedance). In addition, the emitter of Q1 can be back-biased
with respect to the base to provide larger input-signal-handling
capability under full agc conditions.
The operating characteristics of the amplifier shown in Figure 10
are as follows:
Power Gain. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26dB
Agc Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70dB
3dB Bandwidth . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5MHz
Noise Figure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6.8dB
Power Dissipation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7.7mW
The response characteristic is shown in Figure 11.
FIGURE 11. RESPONSE CHARACTERISTIC OF 100MHz AMPLIFIER
OF FIGURE 10
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cautioned to verify that the Application Note or Technical Brief is current before proceeding.
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